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Sedimentary Fabrics of Stratified Slope Deposits at a Site Near U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY SEDIMENTARY FABRICS OF STRATIFIED SLOPE DEPOSITS AT A SITE NEAR HOOVER'S CAMP, SHENANDOAH NATIONAL PARK, VIRGINIA by Joseph P. Smoot 1 Open-File Report 2004-1059 Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1 U.S. Geological Survey, M.S. 926A, Reston, VA 20192 ABSTRACT� An outcrop of stratified slope deposits in Shenandoah National Park is described in detail. The Pleistocene age deposits are comprised of a mixture of clay to cobbles defining a series of offlapping wedges. Elongate clasts are oriented parallel to wedge boundaries except at the toe of the wedge, where they are oriented nearly vertical. The wedges represent sedimentation by freeze-thaw of ground ice. Thin layers of pebbly sand separate matrix-rich wedge deposits, which represent sheetfloods during periods of thaw. Thicker sand layers and lenses of clay are placed upslope of coarse-grained wedge fronts. This association represents ponding of water around the solifluction lobe topography during warm periods. Stratified slope deposits at an outcrop at a higher elevation lack the sandy sheetflood and pond deposits, whereas sheetflood fabrics dominate deposits at a lower elevation. These variations are attributed to differences in temperature at the different elevations. INTRODUCTION Stratified slope deposits are rhythmically layered pebbly deposits that occur on steep slopes with no obvious drainage patterns (DeWolf, 1988). They are attributed to solifluction and sheetflood processes associated with seasonally frozen ground and permafrost. Eaton and others (2003) have described stratified slope deposits from exposures in the Blue Ridge Mountains of central Virginia. The age range of these deposits is within the late Pleistocene as determined from carbon dating of wood 1� fragments (Eaton and others, 2003; Litwin and others, in review). The dates all lie within the age of the last glacial advance or older. This report provides a detailed sedimentological description of one of these exposures with less detailed reference to observations in other localities. These data are used to provide constraints on depositional mechanisms and possible scenarios for the environment at that time. Solifluction is defined as the slow mass movement of water-saturated material down a slope (Washburn, 1980; Lewkowicz, 1988; Ballantyne and Harris, 1994; French, 1996). When the movement is controlled by the freeze-thaw cycles of ground ice, the solifluction is referred to as frost creep and gelifluction. Frost creep is the slow "ratchet" movement of particles down a slope due to expansion of the sediment with ice and the gravitational offset as the ice thaws. Gelifluction is the gravitational sliding of water- saturated sediment as the surface of frozen ground thaws. The underlying frozen ground prevents water movement so pore-pressures increase, leading to failure of the overlying sediment even on low slopes. Rapid movement of a surface layer, called skin flow, is similar to a debris flow. Differential movement of sediment on the surface creates the step-like topography of solifluction sheets and lobes. Isolated boulders may act as obstructions to movement initiating step development. Sediment sorting by frost heave segregates larger grain sizes from finer sediment because finer sediment is entrained in expanding ice more easily. When combined with gravitational segregation of larger grain sizes, this leads to patterns of boulder, cobble, and pebble distributions that are components of the solifluction sheets and lobes. The 2� step-like topography may have relief varying from a few centimeters to several meters on a riser. The fronts of risers may be comprised of tightly packed boulders and cobbles (stone-banked) or they may be comprised of an unsorted mixture of coarse and fine sediment (turf-banked) (Benedict, 1970, 1976). Migration of solifluction sheets and lobes results in aggradation of material, and the front of one feature may overrun that of another. Benedict (1970) noted that the fronts of turf- banked lobes in Colorado moved slower than the axis. Francou and Bertran (1997) noted that the fronts of stone-banked solifluction lobes in Bolivia were moving slower (around 30 cm/yr) than the sheets of fine material behind them (around 100 cm/yr). These observations support the hypothesis that the lobes move in a top-fed fashion, so that the front is continuously buried by the back (Francou, 1990; Van Steijn and others, 1995). Classical gelifluction movement is similar to laminar flow with steadily decreasing velocity from the surface, but plug flow (similar to that of a debris flow) may occur if water saturated-sediment occurs below a frozen surface (Mackay, 1981). This is important in areas of permafrost where two-sided freezing occurs from the base and top of the active layer. In areas of sufficient moisture and summer warming, sheetflooding may wash fine-grained sediment from solifluction deposits, particularly from lobe-fronts of stone-banked lobes. 3� STUDY AREA� The main outcrop described in this report occurs along the headwaters of the Rapidan River in the Shenandoah National Park near Big Meadows (Figure 1) at an elevation of about 2300 feet. This is called the Hoover Camp site because of its proximity to the former U.S. president's cabins. A nearly vertical cut bank, as much as 5 m high, is exposed for about 40 m along the stream edge beneath a bench covered with a fairly recent debris flow deposit. The outcrop is oriented roughly east-west at the base of a southeast-dipping hollow with steeper slopes to the northeast and southwest. Although bedding inclination is variable, there is a general dip to the southwest that is parallel to the steeper portions of the slopes to the northeast. Two additional sites will be mentioned in this report (Figure 1). The first occurs just below the eastern margin of Big Meadows at an elevation of 3500 feet. It is a small gravel pit operated by the National Park Service. The other is a large exposure formed during the 1995 floods along Kinsey Run near the eastern border of Shenandoah National Park, west of Graves Mill at an elevation of about 1200 feet. The latter exposure was described in Eaton and others (2003). 4� METHODS� The Hoover Camp outcrop initially was cleared of loose debris and cut back to nearly vertical using pick and shovel. A rectangular grid framework (Figure 2) was formed using string, with each segment 1 m wide and 1.5 meters high. The surface of each grid segment was carefully scraped clean with a sharp knife (the larger clasts cut into relief), then cleared with a leaf blower. The cleaned grid section was photographed in overlapping 30 cm sections, and then logged in a notebook at a tenth scale. The clasts and other sedimentary features were projected onto a vertical surface for logging with no attempt to correct for bedding dip or the actual dip of the outcrop face. The orientation, sizes, and shape of clasts larger than 5 cm were drawn to scale. Orientation and shape of smaller pebbles and sand grains were noted and indicated schematically. The log of each grid panel was overlapped by 10 cm with adjacent grids to ensure continuity of description. Twenty panels were logged (Figure 3), then combined into a single composite section (Figure 4). Description took several months, so some portions of the outcrop collapsed or became covered, and had to be cleaned again. As a result, some layers and clasts are only approximately aligned in adjacent grids. The uniformity of description was checked using the photos. 5� SEDIMENT DESCRIPTIONS� Grain sizes in the outcrop vary from boulders over a meter in diameter to relatively pure clay. Clasts vary from moderately well rounded to angular in shape. Some angular clasts have rounded sides, and in one place angular fragments of a rounded clast were separated by a small amount of matrix, suggesting fragmentation in place. Clasts are a mixture of granite and metabasalt (greenstone) with minor vein quartz, reflecting rocks in the immediate vicinity (Gaithright, 1976). The clay is predominately kaolinite with less abundant illite that is consistent with local bedrock weathering (Daniel Webster, U.S. Geological Survey, Reston, Virginia, personal communication). Most of the sediment in the exposure contains some portion of muddy matrix comprised of a mixture of clay, silt, sand, and granules. This muddy matrix may occur as pore- filling material in framework gravel and sand or may dominate a sediment layer with randomly distributed pebbles and cobbles. Fine- to coarse-grained sand layers, dominated by rock fragments, quartz, and mica, may be matrix free. The upper contacts of thin sand beds overlain by beds with muddy matrix are often diffuse, suggesting eluviation or mixing of clay-rich material. Beds of nearly pure clay with discrete layers of sand or scattered pebbles and cobbles occur as thin lenses. These clays have a similar composition to those of the muddy matrix, but have a significantly more smectite that is only trace in the matrix (Daniel Webster, U.S. Geological Survey, Reston, Virginia, personal communication). 6� Layering of sediment is defined by varying amounts of muddy matrix, abundance and sizes of clasts, clast orientations, and layers free of muddy matrix. Layers mostly are discontinuous and highly variable in thickness, defining lenses, pods, and wedges. As progressive layers were scraped from the outcrop, it was established that some of the variation in shape is due to different orientations of cross-sections with similar features. A dense root mat extends downward from the forested surface that comprises the top of the outcrop (Figure 3, Grids 1, 5, 8, and 11). Live roots, mostly with diameters of less than one cm; extend more than 50 cm below the surface.
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